An image sensor generally includes an array of pixel cells. Each pixel cell includes a photo-conversion device for converting light incident on the array into electrical signals. An image sensor also typically includes peripheral circuitry for controlling devices of the array and for converting the electrical signals into a digital image.
FIG. 1 is a top plan view block diagram of a portion of a typical CMOS image sensor 10. The image sensor 10 includes an array 11 of pixel cells arranged in columns and rows (not shown). The array 11 includes pixel cells 20 (FIG. 2A) in an active array region 12 and pixel cells 20′ (FIG. 3) in a dark correction region 13 that are used for noise or dark correction. FIG. 2A is a schematic diagram of typical pixel cells 20 and FIG. 2B is a top plan view of a pixel cell 20. The dark correction pixel cells 20′ have the same structure and operate in a similar manner to the active array pixel cells 20. Accordingly, dark correction pixel cells 20′ can be configured as shown in FIG. 2A.
The dark correction region 13 is similar to the active array region 12, except that light is prevented from reaching the photo-conversion devices of the dark correction pixel cells 20′ by, for example, a metal layer, a black color filter array, or any opaque material, depicted as 14 in FIG. 3. Signals from dark correction pixel cells 20′ can be used to determine the dark correction level for the array 11, which is used to adjust the resulting image produced by the image sensor 10, by subtracting the signal generated by the dark correction pixel cells 20′ from the signal from the pixel cells 20, which are used for image capture.
The pixel cells 20 illustrated in FIGS. 2A and 2B are typical CMOS four-transistor (4T) pixel cells. Typically, the pixel cells 20 are formed at a surface of a substrate, as generally shown in FIG. 3. As is known in the art, a pixel cell 20 functions by receiving photons of light and converting those photons into electron charges. For this operation, each one of the pixel cells 20 includes a photo-conversion device 21, which may be a pinned photodiode, but can be a photogate, photoconductor, or other photosensitive device. The photodiode photo-conversion device 21 typically includes an n-type photodiode charge accumulation region 22 and a p-type surface layer.
Each pixel cell 20 also includes a transfer transistor 27, which receives a transfer control signal TX at its gate 27a. The transfer transistor 27 is connected between the photodiode photo-conversion device 21 and a floating diffusion region 25. During operation, the TX signal activates the transfer transistor 27 to transfer charge from the charge accumulation region 22 to the floating diffusion region 25.
The pixel cell 20 further includes a reset transistor 28, which receives a reset control signal RST at its gate 28a. The reset transistor 28 is connected to the floating diffusion region 25 and includes a source/drain region 60 coupled to a voltage supply, Vaa pix) through a contact 23. In response to the RST signal the reset transistor 28 is activated and resets the diffusion region 25 to a predetermined charge level through a supply voltage, e.g., Vaa pix.
A source follower transistor 29, having a gate 29a coupled to the floating diffusion region 25 through a contact 23, receives and amplifies a charge level from the diffusion region 25. The source follower transistor 29 also includes a first source/drain region 60 coupled to the power supply voltage Vaa pix, and a second source/drain region 60 connected to a row select transistor 26. The row select transistor 26 receives a row select control signal ROW_SEL at its gate 26a. In response to the ROW_SEL signal, the row select transistor 26 couples the pixel cell 20 to a column line 22, which is coupled to a source/drain region 60 of the row select transistor 26. When the row select gate 26a is activated, an output voltage is output from the pixel cell 20 through the column line 22.
Referring again to FIG. 1, after pixel cells of array 11 generate charge in response to incident light, electrical signals indicating charge levels are read out and processed by circuitry 15 peripheral to array 11. Peripheral circuitry 15 typically includes row select circuitry 16 and column select circuitry 17 for activating particular rows and columns of the array 11; and other peripheral circuitry 18, which can include analog signal processing circuitry, analog-to-digital conversion circuitry, and digital logic processing circuitry. Peripheral circuitry 15 can be located adjacent to the array 11, as shown in FIG. 1.
In order to obtain a high quality image, it is important to obtain an accurate dark correction level for the array 11. One problem encountered in the conventional image sensor 10 is interference to the signal produced by dark current pixel cells 20′ caused by photons entering the area 12 of the array containing active array pixel cells 20, as shown in FIG. 3, which is a cross-section taken across line X-X of FIG. 1. Dark correction region 13 is shielded from incident light by a shield 14. Longer wavelength light, such as near-infrared or infrared light at 800-1500 μm, may be reflected off the bottom 9 of the substrate 5 and generate carriers B that may also be absorbed by dark correction pixel cells 20′. In addition, when very bright light is incident on active array pixel cells 20 adjacent the dark correction region 13, blooming can occur and excess charge from the active array pixel cells 20, represented by carriers A, can travel to and be absorbed by dark correction pixel cells 20′ in the adjacent dark correction region 13. In addition, excess charge from adjacent circuitry, e.g., peripheral circuitry 15, can travel to and interfere with pixel cells 20′ in the adjacent dark correction region 13.
These sources, and others, cause inaccurate dark correction levels. When enough carriers are absorbed by the dark correction pixel cells 20′, the signal generated by the dark correction pixel cells 20′ will be artificially high, such that the row in active array region 12 corresponding to each of these pixels 20′ will be over-corrected. The row in active array region 12 corresponding to each of the pixels 20′ will have a signal subtracted by a greater amount than actually needed for noise or dark correction. This causes inaccurate dark correction levels, resulting in row banding and distortion of the resultant image. Dark rows may appear in the image, even though they should appear bright in response to a bright subject.
Accordingly, it would be advantageous to have an improved image sensor with reduced interference on dark correction pixel cells.